Generally, non-oriented electrical steel sheets are used for iron cores of motors or generators. These non-oriented electrical steel sheets are classified into electrical steel sheets that require that stress relief annealing (SRA) be performed to improve magnetic properties after punching and electrical steel sheets that do not require SRA to be performed when the costs of heat treatment are excessive in consideration of the increase in magnetic properties due to SRA. With the development of electric appliances, including household electric appliances, the consumption of the non-oriented electrical steel sheets is increasing.
In a process of manufacturing this non-oriented electrical steel sheet, a coating step of forming an insulation film for the interlayer insulation between iron plates is performed in a finishing step. In this case, the insulation film that is formed is required to have basic properties, such as electrical properties for preventing the generation of eddy current, continuous punching workability for preventing the abrasion of a mold when an iron core is fabricated by layering a plurality of steel sheets after punching the steel sheets in a predetermined shape, adhesion resistance for preventing sticking between the steel sheets after SRA of recovering magnetic properties by relieving the stress of the steel sheet, and the like. Further, the coating agent is required to have excellent coating workability and long-term stability after application.
However, as small-sized motors are increasingly used, film characteristics, such as workability, weldability and corrosion resistance, rather than insulation properties, are considered to be important material properties. Recently, since the surface quality of steel sheets has an influence on the properties in use, electrical steel sheets having high surface quality are increasingly required.
Consequently, in order to improve the heat resistance and insulation properties of an insulation film for a non-oriented electrical steel sheet, organic-inorganic composite coating agents, which can make up for the defects of inorganic coating agents, such as phosphate, chromate, and the like, have been developed. Korean Patent Registration Nos. 25106, 31208 and 31219, and U.S. Pat. Nos. 4,316,751 and 4,498,936 disclose methods of forming an insulation film using the organic-inorganic composite coating agent.
Further, Japanese Examined Patent Publication No. Showa 50-15013 discloses a method of forming an insulation film, in which film characteristics, such as lamination factor, adhesivity, punchability, and the like, can be improved by forming an insulation film using a treatment liquid composed of emulsified organic resins, such as dichromate, vinyl acetate, butadiene-styrene copolymer, acrylic resin, and the like, and good film characteristics can also be realized even after SRA.
However, since the above conventional coating agents essentially include chromium oxides, the use thereof may be limited in the light of the current situation, in which environmental standards are strict.
For this reason, methods of preparing a chromium-free coating agent for an electrical steel sheet have been actively developed. The methods of preparing a chromium-free coating agent for an electrical steel sheet may be classified into methods of introducing phosphate into a coating agent to compensate for the decrease in corrosion resistance and adhesivity due to the absence of chromate, and methods of inducing barrier effects by introducing colloidal silica into a coating agent.
In the method of introducing phosphate into a coating agent, as disclosed in Japanese Unexamined Patent Publication No. 2004-322079, the adhesivity and corrosion resistance of the coating agent are improved using the phosphate formed by mixing aluminum phosphate, calcium phosphate and zinc phosphate at an appropriate ratio. However, when metal phosphates are used, the free phosphoric acid present in the metal phosphates may cause the film to be sticky. Therefore, Japanese Unexamined Patent Publication No. Hei11-131250 and Korean Unexamined Patent Publication No. 1999-26912 disclose methods of adding an organic acid and a silane coupling agent to a coating agent to prevent the free phosphoric acid from sticking.
Meanwhile, as typical examples of the methods of inducing barrier effects by introducing colloidal silica into a coating agent, Korean Unexamined Patent Publication No. 1999-26911 and Japanese Patent No. 3370235 disclose methods of forming an insulation film on a steel sheet, in which the corrosion resistance, adhesivity and smoothness of the insulation film are secured after SRA, using inorganic materials, such as colloidal silica, alumina sol, zirconium oxide, and mixtures thereof, and in which the adhesivity and solvent-resistance thereof are improved by adding a silane coupling agent, etc.
Further, Japanese Patent No. 3320983 discloses an electromagnetic steel sheet on which an insulation film is formed, in which the adhesivity and the corrosion resistance thereof are improved by forming a thin dispersion coating thereon when the surface area ratio of resin and silica is appropriate.
However, the above mentioned chromium-free coating agents, which include phosphate or colloidal silica as a main component, have reached a limit in the alleviation of the stickiness thereof due to phosphate, and in the corrosion resistance thereof due to colloidal silica. Therefore, it is still difficult to broadly apply the technology for completely substituting chromium oxides using these chromium-free coating agents.
Meanwhile, in the case where a non-oriented electrical steel sheet is used for an iron core of a motor or a transformer, the non-oriented electrical steel sheets are punched based on standards, layered, and then welded or adhered, thereby obtaining an iron core.
In this work, if necessary, SRA may be conducted. When this SRA is conducted, particularly, the adhesivity, insulation properties and corrosion resistance after annealing become important.
Generally, an insulation film for a non-oriented electrical steel sheet contains chromium. The chromium is helpful in improving the material properties of the insulation film after SRA.
However, in a chromium-free coating agent, phosphate is used as an alternative material for chromium. In this case, the problem with hygroscopicity caused by a very small amount of free phosphoric acid remaining in the film and the problem with adhesivity in annealing due to the hygroscopicity may occur. Paradoxically, these problems can be solved by the introduction of chromium (refer to Equation 1).CrO3+2H3PO4→Cr(PO4)2+6H2O  (1)
Moreover, in a chromium-free coating agent, since the chromium-free coating agent cannot exhibit the compactness of film formed using the same, due to the peculiar film filling effects of chromium compounds, there is a limit in preventing the deterioration of film characteristics.